Project Summary/Abstract The long-term objective of our research is to understand the mechanisms by which obesity, and specifically upper body obesity (UBO), causes insulin resistance and the other health problems such as Type 2 Diabetes. Our focus is on the release of free fatty acids (FFA) from adipose tissue lipolysis in humans and how those FFA affect the functioning of other tissues (lipotoxicity). The specific aims of this proposal are to: 1) Determine whether impaired insulin-induced suppression of lipolysis (as measured by IC50) is related to the above mentioned lipolysis proteins in groups of volunteers known to vary widely with regards to abdominal adipocyte size and regulation of adipose tissue lipolysis; 2) Determine whether the improved insulin regulation of lipolysis resulting from treatment with the PPAR? agonist pioglitazone, with or without weight loss, can be linked to specific changes in sets of PPAR?-responsive adipocyte lipolysis proteins in UBO adults; 3) Determine whether the adipose inflammatory cell and cytokine content in Class III obesity is related to lipolysis insulin resistance and, if so, whether sustained, substantial weight loss one year following bariatric surgery reduces inflammation in parallel with improved insulin regulation of lipolysis. We will measure of insulin- regulated adipose tissue lipolysis in vivo and assess adipocyte lipolysis proteins, inflammation and insulin signaling in obese humans with widely varying degrees of insulin resistance. Our hypothesis is that proteins involved in the final steps of the regulation of lipolysis are altered in humans with large fat cells, and that these alterations are responsible for adipose insulin resistance. Combined, we believe these studies will offer insights as to why adipose tissue lipolysis is abnormal in some obesity phenotypes and provide information on what therapeutic strategies to treat adipose insulin resistance are most compelling.